These mounds are not peculiar alone to Denmark; for they are found in England, Scotland, France, and America. _Danish Peat Bogs._--The peat bogs of Denmark, so faithfully investigated by Professor Steenstrup, mark three periods of deposition. The most ancient is called the _Scotch-Fir_; the second, immediately above, the _Oak_, and the uppermost, the _Beech_. The peat is from ten to forty feet in thickness, and to form a layer from ten to twenty feet thick would require, according to Steenstrup, _at least_ four thousand years, and perhaps even from three to four times that period.[68] These three epochs denote three periods of time. The lowest belongs to the neolithic, the middle to the bronze, and the last to the iron epoch. In the lowest, or _Fir_ period, have been found worked flints and bones. Human bones have been found, which correspond with the bones taken from the tumuli of this epoch. _The Lake-Dwellings of Switzerland._--Dr. Ferdinand Keller and his associates have made known to the world the wonderful remains of villages situated in the lakes of Switzerland and other countries. The villages of Switzerland do not all belong to the same period, and they represent the neolithic, bronze, and iron epochs; but there was no hard line of demarcation between these three periods. These habitations are so numerous that more than two hundred settlements hare been discovered in Switzerland alone. Among the lakes furnishing these remains may be counted the Lake of Neuchâtel (forty-six settlements); Lake Constance (thirty-two settlements); Lake of Geneva (twenty-four settlements); Lake of Bienne (twenty-one settlements); Lake of Morat (sixteen settlements); Lake of Zurich (three settlements); Lake of Pfæffikon (six settlements); Lake of Sempach (six settlements); Lake of Moosseedorf (two settlements); Lake of Inkwyl (one settlement); Lake of Nussbaumen (one settlement); Lake Greiffensee (one settlement); Lake of Zug (six settlements); Lake of Baldegg (five settlements), and others. The habitations belonging to the neolithic are Lake Constance thirty, Neuchatel twelve, Geneva two settlements; one each at Morat, Bienne, Zurick, Pfæffikon, Inkwyl, Moosseedorf, Nussbaumen, the settlement of Concise, the bridge Thiéle, the peat-bog of Wauwyl, and others.